I remember hiring this VHS from the video shop back in the late 80's early 90's. All I can really remember is there was a boy and a girl and they would morph into one person to kill some evil dinosaurs? They would fly around in this jet / spaceship thing. At the very end there would be a dark cave with the evil villian in it with red eyes?

(This might not be true but I think this happened) When the boy and girl merged into 1 person and they were fighting the bad guy of the episode it may have turned into live action footage kinda like power rangers?

In the year 2000, dinosaurs reappear from underground caverns to wreak havoc on the world. Emperor Tyrannus is an evolved dinosaur that plans to dominate the world with his monster dinos.(men in rubber suits)
An elite team of heroes with super advanced technology come to the rescue.
Ai-Zenborg/Attack of the Supermonsters was a typical super sentai type cartoon that featured a team of heroes that piloted a group of super-vehicles, combined with live-action models and effects . Jim the team leader, and his sister Gem had a special power. Each of the siblings was half bionic. When the team wasn't up to defeating the enemy dinos, the two could raise their arms, touch wrists and combine into one giant superhero called Gemini.
(Thunderbirds meet Ultraman)